PROJECT SUMMARY Despite great developments in treatment care for patients with Hemophilia A (HA), still none is curative, and all depend upon lifelong, recurrent, prohibitively expensive, FVIII infusions (?$600,000/year/patient). In addition, >30% of patients with severe HA develop inhibitory antibodies to FVIII, which is the most serious challenge in the clinical management of HA. There is thus a critical need to develop novel therapies that can offer a longer- lasting/permanent improvement, and/or can defeat the immune hurdles that currently thwart treatment. The delivery of FVIII through a cellular platform, such as marrow stromal cells (MSC), is an appealing approach, since following viral transduction, MSC secrete high levels of vector-encoded FVIII that is indistinguishable from the native protein, and do not transform or progress to clonal dominance. In addition, upon infusion, MSC can lodge long-term in multiple organs within both the parenchyma and the perivascular zones, placing them ideally for delivering FVIII into the circulation. MSC have immunomodulatory/anti-inflammatory properties and, if autologous MSC are used, it may enable FVIII delivery in a tolerogenic fashion. We recently tested the therapeutic potential of FVIII-expressing MSC utilizing a line of sheep that emulates the genetics, inhibitor formation (to administered FVIII protein), and clinical symptoms of the severe form of human HA, and showed that, without recipient preconditioning, the postnatal administration of haploidentical MSC engineered to express high levels of FVIII led to complete phenotypic correction of two pediatric HA sheep, reversal of existing hemarthroses, and return to normal physical activity. Remarkably, this phenotypic correction was long lasting despite the presence of high-titer inhibitors in these sheep, and the engrafted MSC were not cleared by the recipient's immune system, enabling them to persist long-term in multiple sites. This prior work leads us to hypothesize that the delivery of FVIII-expressing MSC results in perivascular engraftment and release of FVIII protein into the circulation at levels that could be curative for HA, and that using MSC to stably deliver high levels of FVIII into the circulation may represent a novel means of clinically correcting HA patients with pre- existing inhibitors. Therefore we propose to: 1) Determine the sites and duration of engraftment of FVIII- producing autologous MSC, and test the ability of this therapy to mediate clinical/phenotypic improvement in sheep recipients with and without pre-existing inhibitors; 2) Test whether the immunomodulatory properties of MSC will enable autologous cells to present FVIII to the FVIII-nave recipient in a tolerogenic fashion, such that these animals will be inhibitor-free for life; and 3) Investigate whether the continued delivery of FVIII through this MSC-based approach can be used as a novel means of inducing immune tolerance in animals with pre- existing inhibitors, and compare this method with current immune tolerance induction protocols. We hope that these studies will provide the necessary knowledge for the development of a clinically viable therapeutic strategy to treat/cure severe HA and overcome immunological hurdles in patients beset with FVIII inhibitors.